1. Field of the Invention
The present invention relates to an apparatus and method for reducing turbulence during centrifugal separation of substances. The invention has particular advantages when used in connection with separating blood components using a centrifugal separation channel.
2. Description of the Related Art
U.S. Pat. No. 4,425,112 to Ito, U.S. Pat. No. 4,708,712 to Mulzet, U.S. patent application Ser. Nos. 08/423,578 and 08/423,583, filed Apr. 18, 1995, and U.S. patent application Ser. No. 08/634,167, filed Apr. 18, 1996 (pending), all of which are incorporated herein by reference, disclose a centrifuge used in connection with a tubular blood separation channel. In addition, the following U.S. patent applications identified by serial number, all filed on Jun. 7, 1995, are incorporated herein by reference: 08/480,617; 08/482,285; 08/483,574; 08/484,209; 08/486,012; and 08/504,049. As the channel is spun by the centrifuge, blood flowing through the channel is stratified into components, and ideally each component is then separately withdrawn from the channel through one of a number of outlets in the channel.
In addition to centrifugal forces, other mechanisms may aid in separating blood components in the channel. For example, a groove or passageway in the centrifuge rotor which holds and defines the shape of the channel during rotation, may be formed with sections of varying radii. These changes in radii control flow of particles having varying densities. Components with higher densities will tend to migrate to areas of greater radius.
Another mechanism that may be used to aid in separating components is a dam in the channel. If the dam radially extends from an outer wall of the channel towards the inner wall, it will prevent particles with higher densities from migrating past the dam while permitting lower density particles and liquid to pass between a peak of the dam and the inner wall of the channel. The opposite effect can be achieved by extending a dam from the inner wall of the channel toward the outer wall.
Dams are preferably formed by a protrusion in the channel-holding groove of a centrifuge rotor. When the tubular channel is placed in the groove, the channel conforms to the shape of the groove, and any protrusions in the groove will cause a corresponding dam in the channel.
In one configuration used in connection with separating components of whole blood, the dam may be dimensioned along the entire depth of an outer wall of the channel to prevent red blood cells and white blood cells from flowing past the peak of the dam, while permitting lower density platelets and plasma to pass. A platelet outlet may be arranged in the outer wall of the channel downstream of the dam to collect and separate the platelets from the plasma. This platelet separation occurs because platelets, which have a higher density than plasma, are forced radially outward in the rotating channel, relative to the plasma.
One inefficiency with such an arrangement is that fluid flow over the peak of the dam causes the radial position of platelets and plasma to abruptly change. As the plasma and platelets encounter the dam, their flow is suddenly diverted towards the inner wall of the channel. Once they pass the dam, they sediment outwardly. Such flow condition changes result in "Coriolis" accelerations and decelerations, which in turn cause fairly aggressive mixing of the platelets and plasma to take place. This mixing is counterproductive in a system whose goal is to separate components of flow, and therefore mixing reduces the efficiency of the system.
By mixing platelets and plasma at the outer wall dam, Coriolis effects within the separation channel disadvantageously increase the length of a blood component separation procedure. Reducing blood component separation time is most desirable not only from an economic perspective, but also from a convenience perspective to the donors, who are typically volunteers. The longer the duration of a platelet collection session, the greater the inconvenience to the donor. In addition, when an immediate transfusion is necessary, time may be of the essence.